Project Summary High titers of autoantibodies against synaptic protein NMDAR1 have been demonstrated to cause anti- NMDAR1 encephalitis that exhibits psychosis, fear, anxiety, and other psychiatric symptoms. However, long- term effects of lower titers of anti-NMDAR1 autoantibodies on mental health are unknown, despite that ~5-10% of human population carries low titers of anti-NMDAR1 autoantibodies in their blood. We successfully generated modeling mice carrying low titers of anti-NMDAR1 autoantibodies. Mice carrying the anti-NMDAR1 autoantibodies for many months are healthy, and displayed no behavioral abnormalities except severe deficits in cued fear extinction learning (p=7.80x10-14, effect size: 2.57, power: 0.99) and recall of fear extinction (p=2.29x10-10, effect size: 1.65, power: 0.83), indicating that NMDAR functions may be impaired by the autoantibodies in prefrontal cortex. Impaired fear extinction and recall are clinical endophenotypes for many psychiatric disorders including PTSD, anxiety, schizophrenia, and autism. Peripheral circulating antibodies are largely blocked from entering brain by blood brain barriers (BBB) or blood CSF barriers (BCSFB). However, ~0.1% of blood circulating antibodies can cross these barriers into brain in healthy rodents and humans regardless of antibody specificities. It is conceivable that chronic low titers of circulating anti-NMDAR1 autoantibodies in our preliminary studies could disrupt NMDAR neurotransmission in mouse prefrontal cortex after crossing BBB and BCSFB. Frontal cortex is one of the brain regions innervated with the highest density of blood capillaries, indicating a potential of more influx of circulating anti-NMDAR1 autoantibodies in this region. Since prefrontal cortex plays a central role in the pathogenesis of psychiatric disorders, we hypothesize that prefrontal cortex is particularly vulnerable to chronic presence of low titers of anti-NMDAR1 autoantibodies, and neuroinflammation can exacerbate existing mild NMDAR1 autoimmunity to develop anti- NMDAR1 encephalitis-like phenotypes. In Aim1, we propose to generate mice carrying NMDAR1-Luc2 fusion gene using CRISPR to conduct longitudinal in vivo bioluminescence live imaging (BLI) of NMDAR1 proteins in mouse brain. We expect that NMDAR1-Luc2 proteins will be preferentially reduced in prefrontal cortex by chronic influx of anti-NMDAR1 autoantibodies in mice carrying low titers of anti-NMDAR1 autoantibodies. In Aim2, we will investigate whether systemic inflammation and neuroinflammation may exacerbate existing mild NMDAR1 autoimmunity to develop anti-NMDAR1 encephalitis-like phenotypes. Success of Aim1 will establish a potential mechanistic link between chronic low titers of anti-NMDAR1 autoantibodies that are common in human population and prefrontal cortex in the pathogenesis of a variety of psychiatric disorders. Success of Aim2 will provide evidence supporting that anti-NMDAR1 autoimmunity may be a spectrum of disorders from mild psychiatric disorders to severe anti-NMDAR1 encephalitis.